Census and hospital discharge data from Sweden and Denmark and U.S. Veterans hospitals, linked for follow-up to cancer, mortality, and population registries where available, were used to assess cancer risk among individuals with specific medical and occupational exposures. Mortality among diabetics in Sweden was elevated for all major causes of death, particularly circulatory, respiratory, digestive, genito-urinary, and malignancies. The youngest diabetics had the greatest increased mortality from all causes. We saw the greatest improvement in mortality (34%) in this group during the study period (year of entry 1965-1983), however. Obese Swedish patients were at increased risk of cancers of the digestive tract, urinary tract, female reproductive organs and connective tissue. Swedish psoriasis patients were at increased risk of several cancers associated with alcohol and tobacco consumption, but they had no increase in malignant melanoma. Swedish and Danish acromegaly patients were at increased risk of several cancers, including cancers of the small and large intestine, rectum, brain, thyroid, kidney and bone. Several possible mechanisms could be involved, such as elevated levels of IGF-1 in these patients. Swedish and Danish patients with Down's syndrome (DS) had an increased risk of incident cancers at a young age, particularly acute leukemia, both lymphocytic and non-lymphocytic. Risk of solid tumors was somewhat reduced, except for testicular and digestive cancers. Mortality risks were also elevated for cancer, dementia/Alzheimer's disease, heart disease, cerebrovascular disease, infectious diseases and congenital anomalies. Danish women hospitalized with gonorrhea were at increased risk of cervical pre-neoplasia, but not invasive cervical cancer or any other cancers. Danish patients with hyperparathyroidism had an increased risk of multiple myeloma and cancers of the urinary tract, breast and thyroid gland. In Swedish, Danish and U.S. Veterans hospital data, we investigated the link between inflammatory bowel diseases (ulcerative colitis and Crohn's disease), appendectomy and cancer. We found no evidence of a reduced risk of ulcerative colitis subsequent to appendectomy in either Danish patients or U.S. veterans. In Sweden, children who had undergone appendectomy under age 20 were found to be at no overall increased risk of cancer after an average of 11 years of follow-up; some elevated risks were seen, however, for cancers of the stomach and NHL more than 15 years after appendectomy. Further follow-up of this young cohort is necessary. Swedish celiac patients had an increased risk of lymphoma incidence and cancers of the upper digestive tract. Mortality risks were elevated two-fold overall, but especially increased were lymphoma, auto-immune disorders, and other disorders characterized by immune dysfunction. A large study was initiated to assess familial aggregation of autoimmune diseases and lymphoproliferative malignancies (LP). We are studying 50,000 Swedish and 10,000 Danish cases of LP, twice that number of non-cancer controls, and all their relatives' computerized medical data (cancers and hospital diagnoses). Swedish men and women employed in the painting trades and paint manufacturing were found to be at increased risk of cancers of the head and neck (women) and cancers of the lung, bladder, pancreas and leukemia (men). Swedish women employed in paper packaging, graphic work, dry cleaning, telegraph and telephone work were at increased risk of ovary cancer. Increased risks of melanoma and esophagus, colon, pancreas and brain cancers were observed among Swedish veterinarians. Prostate cancer incidence, but not mortality, was elevated for Swedish men employed in sedentary occupations in 1960 and 1970. Some of these results may reflect the carcinogenicity of occupational exposures, including physical, chemical, viral, and solar or ionizing radiation.